These studies are designed to explore the control of gene expression during the transition from the asexual division mode of reproduction, to the sexual process of conjugation in Tetrahymena. Isotopic labelling of proteins in conjunction with isoelectric focussing and/or SDS slab gel electrophoresis followed by autoradiography of the gel will permit examination of alterations in the pattern of protein synthesis during this switch in biological pathways. Two stages will be intensively investigated: (a) initiation, a mating type independent step induced by nutritional shiftdown, lasting approximately 2 hours, and (b) costimulation, a mating type dependent cell interaction event accomplished by mixing starved mating types, lasting approximately 1 hr, before cells begin to pair. These studies will be complemented by the use of temperature sensitive mating mutants blocked either in initiation or in costimulation. A comparison between wild type and mutant cells will be made of the changes in gene expression in each stage as assayed by electrophoresis. Alterations in cellular morphology and swimming behavior current in response to the nutritional shiftdown induction of initiation and the costimulation interaction initiated by mixing mating types. Cinematic and electron microscopic techniques will be used for these studies. A description of molecular events between external signal and internal alteration in gene expression as well as how new gene products operate to modify behavior and morphology are the ultimate goals of this research.